A variable focal length lens device employing, for instance, a liquid lens system (occasionally simply referred to as a “lens system” hereinafter) based on a principle disclosed in Patent Literature 1 (U.S. Patent Application Publication No. 2010/0177376) has been developed.
The liquid lens system includes a cylindrical oscillator made of a piezoelectric material that is immersed in a transparent liquid. When an alternating-current (AC) voltage is applied to an inner circumferential surface and an outer circumferential surface of the oscillator of the liquid lens system, the oscillator expands and contracts in a thickness direction thereof to oscillate the liquid inside the oscillator. Then, when the frequency of the applied voltage is tuned to an intrinsic frequency of the liquid, a concentric standing wave is created in the liquid to form concentric regions of different refractive indexes around a center axis of the oscillator. When light is introduced into the oscillator along a center axis thereof in this state, the light follows a diffusing or converging path according to the refractive index of each of the concentric regions.
The variable focal length lens device includes the above-described liquid lens system and a focusing objective lens (e.g. a typical convex lens or lens group), which are disposed on a common optical axis. The liquid lens system, which is a packaged liquid lens unit, is installed in the variable focal length lens device.
When a parallel light enters a typical objective lens, the light having passed through the lens is focused at a focus position located at a predetermined focal length from the lens. In contrast, when a parallel light enters the lens system disposed coaxially with the objective lens, the light is diverged or converged by the lens system, so that the light having passed through the objective lens is focused at a position closer or farther than the original focus position (i.e. the focus position without the lens system).
Accordingly, an amplitude of a drive signal (an AC voltage of a frequency forming a standing wave in the liquid inside the lens system) inputted to the lens system is increased or decreased in the variable focal length lens device, thereby controlling the focus position of the variable focal length lens device as desired within a predetermined range (i.e. a range with a predetermined variation width capable of being added to/subtracted from the focal length of the objective lens using the lens system). A sinusoidal AC signal is exemplarily used for the drive signal inputted to the lens system of the variable focal length lens device. When such a sinusoidal drive signal is inputted, the focal length (focus position) of the variable focal length lens device sinusoidally changes. At this time, when the amplitude of the drive signal is 0, the light passing through the lens system is not refracted and the focal length of the variable focal length lens device becomes equal to the focal length of the objective lens. When the amplitude of the drive signal is at a positive or negative peak, the light passing through the lens system is most greatly refracted and the focal length of the variable focal length lens device is most deviated from the focal length of the objective lens.
In order to obtain an image using the variable focal length lens device, an illumination signal is outputted in synchronization with a phase of the sine wave of the drive signal to perform a pulsed illumination. By applying the pulsed illumination to an object while the pulsed illumination is focused on a predetermined value of the sinusoidally changing focal length, the image of the object at this focal length is detected. When the pulsed illumination is performed at a plurality of phases in one cycle and images are detected at timings corresponding to the phases, the images at a plurality of values of the focal length can be simultaneously obtained.
In the above variable focal length lens device, when the pulsed illumination and the image detection are performed at a single phase in one cycle (i.e., a single-plane-image-detecting operation), a detected image (a single-plane detected image) in focus on a single focusing surface (plane) at a focal length corresponding to the phase is obtained.
In the single-plane-image-detecting operation, an image of a part of a surface of the measurement target, in which the part is focused on the focusing surface, is taken as a clear image in focus. However, an image of a part out of the focusing surface (the part is within or beyond the focal length) in the surface of the measurement target is taken as an out-of-focus image.
When the pulsed illumination and the image detection are performed at a plurality of phases in one cycle (i.e., a multi-plane-image-detecting operation), images are sequentially detected at a plurality of values of the focal length corresponding to the phases and sequentially superimposed on each other into a single detected image (a multi-plane detected image). As a result, the image in focus on a plurality of focusing surfaces is obtainable.
However, in the multi-plane-image-detecting operation, although an image of a part of the measurement target, in which the part is in focus on the corresponding focusing surface, can be dearly taken in focus, the image of the part of the measurement target, in which the part is out of focus on the other focusing surfaces, is taken out of focus. Such image information on the focusing surfaces are superimposed on each other, which causes a clear image-quality to be deteriorated into, for instance, an image with blur surroundings of a clear edge.
Against such a deterioration in the image quality, in some cases, the above-described single-plane-image-detecting operation is sequentially performed at a plurality of values of the focal length (i.e., frame-by-frame image detecting operation).
Specifically, the single-plane-image-detecting operation is initially performed at a first value among the plurality of values of the focal length, subsequently the single-plane-image-detecting operation is performed at a second value, and the single-plane-image-detecting operation is repeated by the specified number of the focal length.
In such a frame-by-frame image detecting operation, the image detection at each of the values of the focal length is performed by the single-plane-image-detecting operation, so that the detected images are in focus to provide clear images.
However, since the frame-by-frame mode requires repetition of the single-plane-image-detecting operation by the specified number of the focal length, the image detection time is prolonged as the specified number of the focal length is increased.